The objectives of this thesis are to explore and identify: 1) the causative factors for
extreme endpoints in freshwater chemistry, specifically eutrophication and
acidification, and 2) the convergence of anthropogenic pollution, watershed
composition and climate effects that could contribute to the occurrence of freshwater
chemical extremes. Eutrophication is reviewed as a well-studied water quality issue
that remains relevant as a management challenge. Extra focus is given to acidification,
quantified as a decrease in acid neutralizing capacity (ANC), because it has a strong
influence on physical properties such as nutrient (i.e., phosphorus, nitrogen)
resuspension that can potentially leading to chemical extremes.
Data from lakes in the western Great Lakes region are examined with respect to effects
of acid inputs on in-lake ANC and pH response. Although drainage systems are
discussed, special attention is paid to softwater seepage lakes as being the most
sensitive with regards to acidification risk. The challenges of using data-intensive mass
balance models in lakes with intermittent sampling histories lead to development of a
simpler model for estimating open-water ANC in data-sparse locations. Acid input
sources are compared as combinations of area-weighted charge balances using publicly
available data from long term monitoring programs. Weighted data combinations are
then analyzed using maximum likelihood methods suitable for use with observational
data. The final model correctly predicted low ANC events (ANC < 25 ìeq L-1 ) 20 out of
24 times (R2 = 0.50 adjusted for small sample size; 168 observations), but
underestimates the severity of the lowest extremes.
Three factors stand out as being strongly related to acidification risk during the open
water season: 1) volume of snowmelt, 2) in-lake ANC following spring turnover, and 3) pulsed runoff from associated wetland soils following drought and re-wetting events.
Recommendations for future research focus on quantifying acid and nutrient content
in pulsed runoff events and their impacts on freshwater systems given antecedent
conditions in both lakes and connected wetlands.